The present invention is directed to integrated circuits and their processing for the manufacture of semiconductor devices. More particularly, the invention provides a method and device for avoiding crowbar current during power-on of integrated circuits. Merely by way of example, the invention has been applied to input/output (I/O) interfaces with multiple power supplies. But it would be recognized that the invention has a much broader range of applicability.
Integrated circuits have evolved from a handful of interconnected devices fabricated on a single chip of silicon to millions of devices. Conventional integrated circuits provide performance and complexity far beyond what was originally imagined. In order to achieve improvements in complexity and circuit density (i.e., the number of devices capable of being packed onto a given chip area), the size of the smallest device feature, also known as the device “geometry”, has become smaller with each generation of integrated circuits.
Increasing circuit density has improved performance and complexity of integrated circuits, but it has also introduced additional design constraints. For example, smaller devices generally require low supply voltages, such as 1.2 volts. However, these smaller devices will sometimes interface or interoperate with larger devices of older technology. In such cases, signals can be level shifted to an appropriate voltage level. That is to say, a signal at a 1.2 volts standard can be level shifted to a 3.3 volts standard, or even 5 volts standard, in order for a circuit of typically older technology to properly work. This introduction of multiple supply voltages can lead to crowbar currents, or currents that flows through a short circuit path from a supply voltage to a ground or another supply voltage. Crowbar currents are generally unwanted as devices can be damaged and excess power drained.
In a conventional integrated circuit having multiple supply voltages, a bus coupled to a plurality of I/O pads is particularly prone to unwanted crowbar current. During power-on, one supply voltage may become available before one or more other supply voltages. For example, I/O supply voltage can be powered up prior to a core supply voltage. In such instances, logic states of the plurality of I/O pads coupled to the bus may be unknown. If there is bus contention (i.e., an I/O pad in a high state, logic 1, is coupled to an I/O pad in a low state, logic 0), a crowbar current can develop from one I/O pad to the other.
From the above, it is seen that techniques to avoid crowbar current are desired.